A brilliant show at the Barbican about Marcel Duchamp and his followers reveals how — and why — he changed art for all time

Marcel Duchamp was the most important artist of the modern era. That much is
clear. He was more important than Picasso, Matisse and even Warhol. They
merely changed the direction of art, Duchamp changed its state. Art was one
thing before him, and another thing after him. What is not clear is why he
did it. Why did Duchamp arrive at the radical new definition of art that
constitutes his gigantic contribution to modern culture? Also unclear is why
his changes have proved so popular that much of the art of today owes its
origins to him.

These questions are particularly difficult to answer because we see so little
of his work. Compared with Picasso or ­Warhol, his oeuvre is tiny. A single
museum, the Philadelphia Museum of Art, owns most of what remains and is
understandably reluctant to let it out. What he produced was also
intentionally insubstantial. As